In recent years, improved commercial cracking catalysts have been developed which are highly active for conversion of relatively heavy hydrocarbons into naphtha, lighter hydrocarbons and coke and demonstrate increased selectivity for conversion of hydrocarbon feed, such as gas-oil, into useful liquid products at the expense of gas and coke. One class of such improved catalytic cracking catalysts includes those comprising zeolitic silica-alumina molecular sieves in admixture with amorphous inorganic oxides such as silica-alumina, silica-magnesia and silica-zirconia. Another class of catalysts having such improved characteristics include those widely known as "high alumina" catalysts. Experience gained from using such improved cracking catalysts have shown that maximum benefits from the high conversion activity and improved product selectivity are obtained by maintaining contact of catalyst and hydrocarbon feed for only a limited time of from 1-10 seconds at cracking conditions wherein catalyst is suspended as a dilute phase in a flowing stream of hydrocarbon vapors. Thus, development of these improved fluid catalytic cracking catalyst has led to utilization of dilute phase transfer line reactors wherein a hydrocarbon cracking reaction is carried out with catalyst dispersed in a hydrocarbon vapor stream moving in an elogated reaction conduit with sufficient velocity to keep the catalyst entrained in such vapor as a dilute phase with a minimum of back mixing. Such dilute phase suspensions of catalyst in hydrocarbon vapor may have a density in the range of from 1 to 10 pounds per cubic foot. Accordingly, residence time flexibility inherent to prior art dense phase fluidized bed reaction zones has been sacrificed for the advantages of improved conversion and product selectively obtained with transfer line cracking. However, such residence time flexibility is still desirable for controlling product distribution from a fluid catalytic cracking process.
Cyclic processes for fluid catalytic cracking of hydrocarbon feed streams are well known. The fluid cracking process sequence generally comprises contacting hot regenerated catalyst with hydrocarbon feed in a reaction zone under cracking conditions; separating cracked hydrocarbon vapors from used cracking catalyst; stripping volatile hydrocarbons from said used catalyst with a stripping vapor; regenerating stripped catalyst by burning carbonaceous deposits therefrom with oxygen; and then returning regenerated catalyst for reaction with additional hydrocarbon feed. Hydrocarbon vapors from the reaction step and the stripping step are separated into fractions including a gas product, naphtha, light cycle oil and one or more heavier fractions boiling above the light cycle oil range. Such heavier fractions may be withdrawn as product streams from the cracking process, or may, at least in part, be recycled for further cracking, operating conditions for a fluid catalytic cracking process employing a transfer line reactor having hydrocarbon vapor flowing therethrough with fluidized catalyst dispersed therein as a dilute phase, include regeneration temperatures in the range of about 1,100.degree. to about 1,500.degree.F., regenerator pressure (above the fluidized bed) in the range of about 5-50 psig, transfer line reactor outlet temperatures in the range of 850.degree.-1,200.degree.F., preferably 925.degree.-1,000.degree.F. or higher; reaction zone pressures in the range of 5-50 psig; catalyst to oil weight ratios in the range of 2-20 pounds of catalyst per pound of oil. The cross-sectional area of the transfer line reactor is selected to provide superficial vapor velocities of 15-25 feet per second at the transfer line inlet and of 20-60 feet per second near the outlet. Residence time of reactant vapors in the reaction conduit are preferably in the range of 0.5-10 seconds. Combinations of the above operating conditions may be employed to obtain a hydrocarbon feed conversion in the 60-95 percent range, preferably 75-85%, wherein hydrocarbon feed conversion is defined as that percentage of the hydrocarbon in the feed boiling above about 430.degree.F. which is converted to coke and hydrocarbons boiling below 430.degree.F.
At different seasons, it is often desirable to vary product distribution obtained from a fluidized catalytic crackiing unit. Particularly, it is often desirable to vary the ratio of naphtha to light cycle gas oil obtained. The degree of conversion, which affects such product ratios, is conveniently controlled by adjusting reaction conditions, particularly residence time of catalyst and hydrocarbon vapor in the reaction zone.
Fluid catalytic cracking apparatus is known which employs a transfer line reactors for dilute phase hydrocarbon cracking processes, which reactors are designed to provide flexibility of catalyst-hydrocarbon mixture space velocity under cracking conditions. For example, in U.S. Pat. No. 3,644,199, apparatus and a process are disclosed wherein catalyst-hydrocarbon vapor mixture is admitted into the lower portion of a vertical transfer line reactor comprising an external pipe and an internal concentric pipe open at its lower end and which may be closed at its upper end. The external pipe is fitted with catalyst-hydrocarbon inlet means near its lower end and outlet means near its upper end. The open lower end of the internal concentric pipe terminates above such catalyst-hydrocarbon inlet means and the internal concentric pipe upper end terminates outside the closed upper end of the external pipe. Valve means are provided for closing the upper end of the internal concentric pipe in a controllable manner. Velocity and residence time of catalyst-hydrocarbon mixture in said transfer line reactor are varied by controlling the degree of closure of the upper end of the internal concentric pipe. That is, with the internal pipe closed, catalyst-hydrocarbon mixture can only flow through the annular area between the external and internal pipes; with the internal pipe open, catalyst-hydrocarbon mixture flows at lower velocity and longer residence time through the annular area and the internal pipe. Intermediate velocities and residence time may be obtained by adjusting the degree of closure of the inner concentric pipe.
The disadvantage of this apparatus is that velocity of the catalyst-hydrocarbon mixture varies with the degree of closure of the inner concentric pipe. Under certain flow conditions, the velocity may drop so low that substantial backmixing of catalyst in the hydrocarbon vapor stream may occur, leading to undesired loss in naphtha selectivity and an increase in coke production.